The present invention relates to a method for the continuous extrusion of a metallic material, so that the amount of non-extruded material as well as drawbacks caused by thermal expansion can be reduced, and at the same time the working life of extrusion tools extended.
Continuous extrusion often applies a method described in the GB patent 1370894 and called the Conform method. In this method, the material to be extruded is conducted in a groove formed on the outer circumference of a wheel-like member. While said member rotates around its axis, the material to be extruded gets into contact with a cooperating shoe filling the groove, so that the motion of the material to be extruded with respect to the wheel-like member changes. Thus the material is fed to extrusion in the proceeding direction thereof, either prior to the cooperating shoe or through an extrusion aperture located in said member.
The metallic material to be extruded can be fed in a groove according to the Conform method in a granular, finely divided, molten or solid form. When using a granular or finely divided feedstock, the material has a large specific surface, in which case it is difficult to remove possible surface oxidation defects in the extruded product owing to the large surface area. When using solid, for instance rod-shaped feedstock, the force required for extrusion increases, but the defects caused by surface oxidation are, however, reduced. A solid, rod-like feedstock can be protected against oxidation for instance by means of a protecting hood described in the international WO patent application 95/17270.
The forces used in the extrusion of metallic material are remarkable, which leads to a rise in temperature both in the extrusion tools and the material to be extruded. This temperature rise can be prevented for example by following the method introduced in the U.S. Pat. No. 4,610,725, i.e. by conducting a cooling agent into the groove both through the feedstock feed aperture and through a specific nozzle conducting additional cooling agent directly to the groove. Moreover, according to the U.S. Pat. No. 4,610,725, the increase in temperature is prevented by feeding some cooling agent into the cooperating shoe closing the groove. This type of cooling is sufficient for instance with non-alloyed metals and extrusion products with simple transversal surfaces. When using a cooling agent, space must also be reserved for the removal thereof, which in part increases the production and maintenance costs of the apparatus.